Dual-photon absorptiometry is being examined and developed for measurement of total body bone mineral in vivo. The method involves scanning with an external sealed source (1 to 2 Ci of 153-Gd). The transmitted radiation is detected with a scintillation detector. The source and detector are passed in a retilinear raster pattern across the entire body during the course of about 70 minutes while the subject lies comfortably on the scanning table. These scan measurements permit the quantitation of the mass of mineral and the mass of soft-tissue at each point in the body; at points where there is no bone mineral the relative lean-fat composition of the soft-tissue is determined. We have acquired a graphics terminal and can now output the data in the form of quantitative images which show the distribution of fat, lean-tissue, and/or bone mineral. This also allows one to determine the composition of different anatomical areas of the body. This new technique has been evaluated in our laboratory and the sources of error have been parameterized. The method has been cross-validated against total body calcium determinations in vivo (by neutron activation analysis). We have also done measurements of the trunk mineral (60% of the total) and cross-validated these against trunk calcium (by neutron activation). Studies have been done in 60 normal women over age 50 to define normal values. Longitudinal studies are being done in patients with renal osteodystrophy, particularly with reference to vitamin-D treatment. Measurements of the 24-hour uptake of 99m-Tc-HEDP have been done using a whole-body counter. These studies have been done in conjunction with absorptiometry. A higher than expected bone uptake was found in renal patients and in patients with prostrate cancer.